In the modern era of ubiquitous computing, privacy is one of the most critical user concerns. To prevent their privacy, users typically, try to remain anonymous to the service provider. This is especially true for decentralized Peer-to-Peer (P2P) systems, where common users act both as clients and as service providers. Preserving privacy in such cases requires mutual anonymity, which shields the users at both ends. Most unstructured P2P systems like Gnutella, Kazaa provide a certain level of anonymity through the use of a random overlay topology and a flooding based routing protocol, but suffer from the lack of guaranteed lookup of data. In contrast, most structured P2P systems like Chord, are Distributed Hash Table (DHT) based systems and provide guarantees that any stored data item can be found within a bounded number of hops. However, none of the existing DHT systems provide any mutual anonymity.

In this paper, we present Agyaat - a decentralized P2P system that has the desired properties of privacy-preserving mutual anonymity and still accomplishes the performance benefits of scalable and guaranteed lookups. A unique characteristic of its design is its low-cost, yet highly effective approach to support mutual anonymity. Instead of adding explicit anonymity services to the network, Agyaat advocates the utilization of unstructured topologies, referred as clouds, over structured DHT overlays. Cloud topologies have an important feature of local query termination, which is critical to facilitate mutual anonymity. To overcome the drawbacks of typical Gnutella like systems, Agyaat introduces a number of novel mechanisms that enhance the scalability and efficiency of routing. Compared with existing pure DHT based systems, Agyaat provides mutual anonymity while ensuring similar routing performance (differing only by constants) in terms of both number of hops and aggregate messaging costs. We validate the Agyaat solution in two steps. First, we conduct a set of experiments to analyze the system performance and compare it with other popular pure DHT based systems. Second, we perform a thorough security (anonymity) analysis under the passive logging model. We discuss possible privacy compromising attacks and their impact, and propose various defenses to thwart such attacks.